§2.1 Field of the Invention
The present invention concerns whispering gallery mode (“WGM”) sensors that can detect the presence of, identify the composition of, and/or measure an amount or concentration of substances (referred to generally as “target entities,” “target analytes,” or simply “targets”), such as chemical or biological entities. The present invention also concerns methods and apparatus to make, and/or use such WGM sensors.
§2.2 Background Information
There exists an ongoing need for sensors for detecting various targets such as, for example, infectious agents (e.g., viruses, bacteria, etc.), toxins, small amounts of proteins, DNA, RNA, etc. Similarly, there exists an ongoing need for sensors for measuring DNA hybridization, protein adsorption, biomolecular mass, etc. Dip sensors would be convenient for interfacing with fluid samples. However, they are not available for all sensing methods. If available, the sensitivity is often compromised compared with non-dip sensors.
Nearly all biosensing methods that claim a high sensitivity, such as surface plasmon resonance and quartz crystal microbalance for example, use a microfluidic system for (1) introducing a sample solution for analysis and (2) rinsing the sensor. Unfortunately, this is much more complicated compared with introducing a dip sensor into a sample solution and subsequently rinsing the sensor in a target-free buffer. Such known high sensitivity biosensing methods have not been used as dip sensors because, in each case, their sensor head assembly is too large to match the size of the wells in well plates.
The need for fast and early detection of pathogens (e.g. viruses) and the antibodies that are generated as a biological response has led to the development of ultra-sensitive label-free biosensors that can detect individual bio-nanoparticles in aqueous solution. One known device used to detect the presence of small particles is a microsphere sensor coupled with an optical waveguide (e.g., an optical fiber with an eroded section), one end of which is optically coupled with a light source and the other end with a light detector. Whispering gallery modes (“WGM's”) of the light circulating within the microsphere can be observed in optical signals detected at the detector. Examples of such WGM sensors are described in U.S. Pat. No. 7,491,491 (referred to as “the '491 patent” and incorporated herein by reference).
WGM sensors rely on the inherent sensitivity of the WGM resonances within the resonator to changes in the external environment to provide a sensitive detection mechanism. When light travels within a transparent medium of a circular cross section on a track near the surface of the medium by total internal reflection, the light superimposes onto itself after one cycle. If the cycle contains an integral number of waves, the superposition is constructive, and the amplitude of light multiplies by hundreds or thousands. (See, e.g., 100 of FIG. 1.) This strong mode of light propagation is observed as a narrow-line resonance called a “whispering gallery mode” or “WGM”.
The wavelength is sensitive to the temperature, as the refractive index of the resonator changes with temperature. The resonance wavelength is also sensitive to the refractive index of the immediate neighborhood of the surface. Target entities selectively captured (e.g., adsorbed) by target receptors onto the surface of the microsphere (or some other resonator) may shift the whispering gallery modes. As can be appreciated by comparing FIGS. 2A and 2B with FIGS. 2C and 2D, a WGM sensor 200 utilizes the shift of resonance wavelength (Compare FIGS. 2A and 2C.) when a receptor 220 binds to a ligand 210 immobilized on the resonator's surface (Compare FIGS. 2B and 2D.). Since an extremely small shift can be easily observed, the resonator has a high sensitivity to the temperature and the composition of the surrounding medium.
After being proposed more than ten years ago, there has been intensive research on WGM sensors. WGM sensors have emerged as an important optical tool for detecting and analyzing trace quantities of biological materials. WGM sensors have been employed in a host of applications including the detection of virus and bacteria, measurement of DNA hybridization and protein adsorption, and biomolecular mass determination.
U.S. Patent Application Publication No. 2004-0137478 (referred to as “the '478 publication” and incorporated herein by reference), titled “ENHANCING THE SENSITIVITY OF A MICROSPHERE SENSOR,” discusses increasing the sensitivity of WGM sensors. More specifically, the '478 publication describes creating a band (e.g., a narrow band) of target receptors such that the target receptors are substantially limited to a highly sensitive region near the equator of a microsphere (also referred to as the “equator region”). The '478 publication discusses fabricating microsphere sensors having target receptors substantially only at a sensitive equator region of a microsphere's surface.
U.S. Pat. No. 8,642,111 (referred to as “the '111 patent” and incorporated herein by reference), titled “FUNCTIONALIZING A SENSING RIBBON ON A WHISPERING GALLERY MODE MICRORESONATOR USING LIGHT FORCE TO FABRICATE A WHISPERING GALLERY MODE SENSOR,” discusses using light force to fabricate WGM sensors including microresonators having target receptors selectively and substantially provided at only equator region (or mode volume) of the microresonators. More specifically, the '363 publication discusses fabricating microsphere sensors for determining the presence or concentration of a target entity in a medium.
U.S. Pat. No. 8,493,560 (referred to as “the '560 patent” and incorporated herein by reference), titled “PLASMONIC ENHANCEMENT OF A WHISPERING-GALLERY-MODE BIOSENSORS,” describes sensors for determining the presence or concentration of a target entity in a medium. Such sensors may include (a) an optical waveguide; (b) a microresonator optically coupled with the optical waveguide such that light within the optical waveguide induces a resonant mode within the microresonator at an equator region (or a mode volume); and (c) at least one plasmonic nanoparticle adsorbed onto a surface area of the microresonator within the equator region (or the mode volume) such that light inducing a resonant mode within the microresonator also causes a plasmonic resonance in the at least one plasmonic nanoparticle. Detection methods for using such sensors are also described. Finally, methods, involving the use of carousel forces, for fabricating such sensors are also described.
U.S. Pat. No. 8,886,270 (referred to as “the '270 patent” and incorporated herein by reference), titled “SYRINGE-BASED WHISPERING GALLERY MODE MICRORESONATOR MICROFLUIDIC BIOCHEM SENSOR,” describes a syringe-based whispering gallery mode sensor having a syringe, the syringe including an assembly provided its needle, the assembly including (1) an optical carrier having a reflective distal end, and (2) at least one resonator coupled with the optical carrier. This sensor may be provided in a system including a light source, a light detector, and a data analysis component. A method for determining the presence or concentration of a target substance in body fluid may be performed using such a system.
As shown in FIG. 3, past studies of WGM biosensor and chemosensor used, for example, a fluidic device 300 that holds a fluid permeating the resonator and a taper (to feed light into the resonator). When a fluid containing target molecules is introduced into the surroundings, the resonance wavelength shifts. (Recall FIGS. 2A-2D.)
In the fluidic device, the position of the taper relative to the resonator is carefully optimized by a three-dimensional positioner. Unfortunately, however, common bench top noises can easily disturb the coupling and extinguish the resonance. To reestablish resonance, adjustment is needed to reposition the taper relative to the resonator. The requirement of frequent optical repositioning has been a source of frustration for biologists, clinical scientists, biochemists and chemists who want to study and/or exploit WGM sensors. Furthermore, such repositioning can cause scratches on the resonator's surface, and such scratches can make the resonator useless in a WGM sensor. Consequently, the past studies have been mainly conducted, nearly exclusively, in physics, bioengineering, and electrical engineering laboratories.
Therefore, it would be useful to provide a more robust WGM sensor. A ready-to-use, mechanically robust, pre-assembled sensor head that does not need optical alignment would be especially useful. It would be useful if such a WGM sensor could be dipped into, and removed from, wells of known well plates, without losing or disturbing its resonance.